Mouse leukemic cells (L5178Y), human cells (HeLa), and several types of Chinese hamster cells from different tissue sources (V79 lung, M3-1 bone marrow, CHO ovary), will be labeled with precursors of DNA containing tritium, C14, and I125, or tritiated precursors of protein and RNA. After being synchronized by various methods, these cells will then be cooled to 4 degrees C or frozen to -196 degrees C to accumulate damage due to tritium and other isotope disintegration. After suitable time intervals the cells will be thawed out and the reproductive ability and chromosome damage will be determined. The efficiency of cellular inactivation and induction of genetic damage will be compared for damage accumulated in various subcellular regions. These experiments are designed to further explore regions within the mammalian cell nucleus where isotopic decay appears to be most critical in the induction of reproductive death and chromosome damage. We are particularly interested in the DNA which replicates late in the cell cycle, including some of the sex chromosomes and DNA near the nucleous membrane, as well as tritium damage in satellite DNA. Two other experimental parameters which appear to be quite important, the time in the cycle of the accumulation of damage, and the quality and the repairability of the damage that is formed, will also be studied. The results of these studies may provide information of two general types: 1) information on the basic mechanisms which lead to the inactivation of proliferative ability of mammalian cells and the induction of chromosome aberrations, and 2) quantitative estimates of the relative hazards to the population from environmental tritium released to the biosphere as a by-product of nuclear power production.